from Wiktionary, Creative Commons Attribution/Share-Alike License

Etymologies

double (twice, i.e. more than) + Dutch For sense 1: The English, historically, found Dutch (albeit a fellow Western Germanic language) particularly difficult to understand, so anything that was completely incomprehensible would be double Dutch, i.e. twice as hard as Dutch. (Note that historically, "Dutch" could refer to German dialects, as well.) (Wiktionary)

A double Dutch is something you just don't get.. It's like if a NASA scientist's telling you, a regular fella, how to launch a rocket somewhere.. Eventually, you might say: this is double Dutch! I just can't get wathever you're trying to say!!